Due to the rise in energy prices in mind and the entry into force of the Kyoto Protocol, a cogeneration type energy supply system for combined heat and power has been adopted in Japan to reduce greenhouse gas. For conventional thermal power generation dedicated to power generation only, on the other hand, there has been new technology developed like ultra super critical power generation having a thermal efficiency boosted up by increasing in steam temperature and steam pressure, followed by the development of newer technology for an additional thermal efficiency (Non-Patent Reference 1). That is, in the ultra super critical power generation having excellent thermal efficiency among conventional power generations, it is now said that the steam temperature maxes out at about 630° C. and the thermal efficiency maxes out principally at 42 to 43% (sending-end higher heating value: HHV) too. With recently advanced materials technology, however, a possibility of achieving steam conditions of at least 700° C. and at least 24.1 MPa steam pressure is now in sight. This emerges in the form of a plan of developing advanced ultra super critical (A-USC) power generation taking advantage of these materials, making sure energy security and reducing CO2 emissions for the purpose of environmental friendliness.
A-USC is technology capable of achieving high thermal efficiency (sending-end HHV): 46% at a steam temperature of 700° C.-class, 48% at 750° C.-class and 49% at 800° C.-class and there is an immediate mounting demand for this technology with a view to being well compatible with replacement of long-standing thermal power generation plants, which demand will mount up after 2020. As part of this, materials resistant to high temperatures are now under development (Non-Patent References 2 and 3).
It is here to be noted that such materials resistant to high temperatures may be used not only in the aforesaid A-USC application but also for conventional thermal power generation and a variety of energy supply installations at a steam temperature of 600° C.-class.
For instance, high-strength ferritic heat-resistant steel is known as one of such materials resistant to high temperatures, and Non-Patent Reference 4 as an example makes a reference to the materials standard for high-strength ferritic heat-resistant steel: KA-STPA29 (alloy steel pipes for power generation piping) and KA-STBA29 (alloy steel tubes for power generation boilers). For instance, Non-Patent Reference 5 is also known for another standard for those materials standards. In the ASME (American Society of Mechanical Engineers) boiler and pressure vessel code, however, retrofitting of Grade P92 and Grade T92 that are equivalents of KA-STPA29 and KA-STBA29 to standardized plants is held in abeyance under present circumstances, because of some considerable lowering of their creep rupture ductility. To keep the reliability and safety of structural members used under high-temperature and high-pressure conditions, it is required to hold back the degree of lowering of creep rupture ductility. Thermal power generation plants must be run with rapid and frequent load changes in association with the widespread use of sustainable energies such as wind power and solar power. Temperature changes incidental to such load-changing operation reduce the service life of high-temperature structural members accompanying repetition of thermal expansions; however, improvements in creep rupture ductility may help prevent service life from getting short due to repeated thermal expansions.
In other words, it is known that high-strength ferritic heat-resistant steels have improved creep strength at high temperatures, but their creep rupture ductility deteriorates considerably under conditions under which they are used over an extended time period, leading to concern that there may be damages done to the safety and reliability of high-temperature structural equipment. Possible causes of poor creep rupture ductility may include the formation of voids due to coarse precipitates or nonmetallic inclusions, influences of impurity elements, and such.
Such Non-Patent References 6-9 as mentioned below are known in terms of improvements in the creep strength of high-strength ferritic heat-resistant steels, but they are quite silent about creep rupture ductility that is an important feature for compatibility with demands for replacement of long-standing thermal power generation.
That is, Non-Patent Reference 6 teaches that the creep strengths of high-strength ferritic heat-resistant steels are improved by thermo-mechanical treatment. As taught, creep strengths are improved by thermo-mechanical treatment in a single austenite phase temperature region in which a second phase that is a strengthening factor is finely dispersed and precipitated, but it remains unclear whether or not the creep rupture ductility is improved under the heat treatment conditions disclosed therein.
Non-Patent Reference 7 teaches that normalizing heat treatment is carried out at a temperature higher than provided in the ASTM standards and tempering heat treatment is carried out at a temperature lower than provided in the ASTM standards thereby improving the creep strengths of high-strength ferritic heat-resistant steels. This is done for the purpose of carrying out normalizing heat treatment at a temperature higher than usual and tempering heat treatment at a temperature lower than usual to effect fine dispersion and precipitation of a second phase that is a strengthening factor thereby improving creep strength; however, it has yet to be clarified whether or not the creep rupture ductility is improved by the heat treatment conditions disclosed therein.
Non-Patent Reference 8 shows the results of studies made of influences of a composition partitioning between partially transformed martensite and untransformed austenite of Si—Mn steel on its mechanical nature. As taught, carbon migrates from martensite to untransformed austenite so that a high-carbon austenite phase remains in the form of residual austenite, resulting in improvements in strength-ductility balance; however, whether or not the creep rupture ductility is improved by the heat treatment conditions disclosed therein remains unclear.
Non-Patent Reference 9 teaches that modified 9Cr-1Mo steel that is a high-strength ferritic heat-resistant steel is partially transformed into martensite and then heat-treated for tempering without being cooled down to room temperature. As taught, the size of precipitates that are a strengthening factor is reduced to increase the size of a martensite block thereby improving creep strengths; however, whether or not the creep rupture ductility is improved by the heat treatment conditions disclosed therein has yet to be clarified.
On the other hand, Patent References 1 and 2 come up with improving the creep strengths of high-strength ferritic heat-resistant steels by addition of Ti and normalizing heat treatment at a temperature higher than usual; however, it remains unclear whether or not there are improvements introduced in creep rupture ductility under the conditions for chemical compositions and heat treatments of the disclosed high-strength ferritic heat-resistant steels.